Your expansion tank is either defective or waterlogged depending on which type you have. At 25 years I would think you have a long horizontal cylinder for an expansion tank and if so it needs to be completely drained so that it can refill with air.

: I wrote on Dec. 30, 2003 that my gas fired baseboard hot water furnace had water leaking from the pressure relief valve. I followed the advice and replaced both the pressure relief valve and the feed valve. The leak stopped for 3 weeks. First a few ounces of water and now about a gallon per day. The furnace is 25 years old. When the furnace is off, the pressure reads 12psi. When the furnace is running the pressure is 35psi and when it reaches that level, the water begins dripping.